In a battery system, a plurality of battery cells can be connected in series to increase an output voltage and in parallel to increase charging and discharging current. For example, a large-current and high-output battery system used as a power source for a motor that drives a vehicle has a plurality of battery cells connected in series to increase an output voltage. In a battery system to be used in this application, a plurality of battery cells is connected by a busbar made of a metal plate. The busbar is connected to electrode terminals of the battery cells constituting the battery system by laser welding. In this connection structure, the busbar is provided with a cut-away portion, and the electrode terminals of the battery cells are inserted into the cut-away portion. A boundary between the inserted electrode terminals and the busbar is irradiated with a laser beam, and both the electrode terminals and the busbar are melted and connected to each other at the boundary therebetween. This connection structure has a problem in that when a gap is generated between the busbar and the electrode terminals to be laser-welded, reliable and stable welding cannot be carried out. This is because metals of both the busbar and the electrode terminals cannot be melted by irradiation with the laser beam. The laser welding is carried out by irradiating the boundary between the busbar and the electrode terminals with a laser beam in a state in which the busbar and the electrode terminals are closely attached to each other so as to weld-join the both metals to each other. However, in a battery system including a plurality of battery cells, it is very difficult to eliminate a gap between a cut-away portion of a busbar and electrode terminals because of a dimensional error in the battery cells and/or the busbar. In the cut-away portion of the busbar, the electrode terminals of adjacent battery cells are inserted, and the battery cells are connected in series or in parallel. Accordingly, at least two electrode terminals of the battery cells are connected to one busbar. An interval between the electrode terminals of the adjacent battery cells varies depending on a dimensional error in the battery cells, and therefore the interval does not become constant. In order to electrically connect electrode terminals whose interval is not constant by one busbar, the busbar absorbs the dimensional error of the battery cells by forming an elongated hole as a cut-away portion into which columnar electrode terminals are to be inserted.
In a structure in which a columnar electrode terminal is inserted into an elongated hole, an entire periphery of the electrode terminal cannot be brought nearer to the inner edge of the elongated hole, and a gap is generated in part. This gap inhibits stable laser welding. In order to prevent this problem, a battery system in which a welding ring is stacked to a busbar, and the welding ring is laser-welded to the electrode terminals and the busbar is developed (see Patent Literature 1).